It is desirable for engine manufacturers to have a means for quickly determining engine oil consumption, because oil usage is important, not only for reasons of customer satisfaction, but also as a measure of basic engine integrity.
Many methods have been proposed for measurement of engine oil consumption. U.S. Pat. No. 3,473,372 to Klink discloses a system which uses a calibrated measuring vessel for determining oil usage. Such a system is hardly much of an improvement over the oldest known methods for measuring oil consumption in which the engine being tested was merely operated for an extended period of time sufficient to allow weighing or volume measurement techniques to determine, albeit with mediocre accuracy, the engine's oil consumption.
Engine designers have sought improved ways for measuring engine oil consumption for many years. One such alternative has involved the measurement of trace compounds in the engine exhaust. For example, a radiometric method involves the addition of a radioactive tracer to the oil, with the tracer being tracked in the engine's exhaust. This technique suffers from the drawback that it requires synthesis and addition of the radioisotope tagging compound, which renders the technique generally unsuitable for routine use.
Another method for determining engine oil usage involves the measurement of an oil additive, such as zinc dialkyldithiophosphate. Such a method is disclosed in U.S. Pat. No. 4,321,056 to Dimitroff. As disclosed by Dimitroff, a sample of the exhaust gas in the engine is passed through a condenser in order to condense zinc sulphate in the exhaust. After the sample is treated, it is passed through a coulometer cell wherein a reading is obtained which is proportional to the engine oil consumed during the sampling period. This system, unfortunately, is incapable of giving a real time measurement of engine oil consumption.
Electrochemical cells for measuring trace chemical constituents are also disclosed in U.S. Pat. Nos. 3,928,162 to Takata; 4,029,563 to Binder et al; 4,409,069 to Luft; and 4,622,105 to Liu et al. None of these cells is suitable for the continuous measurement of sulphur dioxide or any other trace element carried in the stream of exhaust coming from an engine, on a real-time basis.
U.S. Pat. No. 4,277,368 to Amy et al and U.S. Pat. No. 4,499,190 to Spicer et al disclose coulometric and fluorescent techniques for detecting sulphur dioxide. As with the previously noted coulometric methods, these methods are not suitable for measuring sulphur dioxide in a flowing exhaust stream from an engine in real-time because they lack adequate time response characteristics.
U.S. Pat. No. 4,500,391 to Schmidt discloses an electrochemical detection cell and circuitry capable of applying a fixed DC voltage bias to the reference electrode and superimposing a train of DC voltage pulses on the fixed bias. As disclosed by Schmidt, the circuitry functions to determine the difference between the sensing electrode signals. The difference is proportional to the concentration of the test gas in the atmosphere adjacent to the sensor.
U.S Pat. No. 4,735,691 to Green et al discloses an electrochemical detector cell consisting of two electrodes in which the cell is connected to an external current measurement circuit through a cyclically operated switch which alternately opens and closes a connection between the cell and the external cell. The cell is operated without bias potential. According to Green, by taking the difference between cell amplitudes, a differential current sample is provided which should be indicative of the cell current signal alone.
U.S. Pat. No. 4,642,172 to Fruhwald discloses a bias circuit adapted to be coupled with an electrochemical fuel cell for use in the detection of carbon monoxide or hydrogen sulfide.